The main objectives of this study are: 1) determine the dose-response range of Norwalk virus (NV) infection in adult human volunteers and 2) begin to define whether ondansetron may be a potential treatment for acute viral gastroenteritis. It's likely that a significant portion of "acute gastrointestinal illness of unknown etiology" episodes are related to NV, & that Norwalk and related viruses may be responsible for up to 23% of waterborne outbreaks of acute gastronenteritis. Use of ondansetron, may not only ameliorate symptoms of infection, but may also prevent spread of infection by deceasing vomiting episodes.